When examining the impacts of invasive species, plants are often overlooked, despite causing significant ecological, agricultural, economic, and even human health harm. Invasive plants are plants that have been introduced by humans into a novel environment in which they did not evolve and where very few factors which limit their growth and reproduction exist (Westbrooks 1998). Once established, invasive plants cause serious ecological harm and incur economic costs (Westbrooks 1998; Ranney & Ranney 2004). Some invasive plants even impact human health, either directly, such as by poisoning, or indirectly, by providing habitat for or attracting hosts for organisms that carry diseases afflicting humans, such as ticks. Most of the invasive plant species in the United States were introduced for horticulture purposes (Harris et al. 2009). The impact of invasive plants on ecosystems, natural resources, and managed lands is expected to continue to increase with climate change (Bradley et al. 2010; Turbelin & Catford 2021). Climate change will create more favorable conditions for invasive plants to expand into new ranges that were previously inhospitable (Bradley et al. 2010).
Horticulture and nonnative plants
Horticultural practices are a significant contributor to the spread of invasive plant species worldwide historically and today. Most invasive plants were initially introduced to the United States for horticultural purposes. Horticulture continues to be a source of plant invasions with nurseries playing a major role in the introduction of non-native plants (Ranney & Ranney 2004; Harris et al. 2009). The vast majority of the invasive plants in the United States were introduced intentionally as ornamental plants with only a few being accidentally introduced through commerce activities. Most invasive plants have been introduced repeatedly, colonizing new areas with each introduction and increasing the likelihood that the plant will become invasive.
Many of the exotic non-native plants sold in nurseries can easily become invasive in new territories without the limiting factors present in their native range. Once established, invasive plants reproduce rapidly, displacing native plants, and disrupting ecosystems (Ranney & Ranney 2004). The nursery industry is a significant contributor to invasive plants in the United States. Over 85% of invasive woody plants were initially introduced for ornamental use in landscaping (Ranney & Ranney 2004). Repeated introductions of nonnative plant species in lawns, gardens, and landscaping have sped up invasions (Harris et al. 2009).
Family membership and native origin are common indicators of the potential of a plant to become an invasive species in the future (Harris et al. 2009). Species that belong to certain families and come from certain parts of the world have a higher probability of being introduced to new areas through horticultural use. A study of 462 horticultural plants carried by nurseries concluded that native origin and family membership were effective indicators to identify invasive species (Harris et al. 2009; Boyce 2010). Species of plants from Asia tend to be most popular, most likely to be carried by nurseries, and also have the highest potential invasiveness (Harris et al. 2009).
Urban and suburban ecosystems tend to be “hotspots” of invasions due to horticultural practices, the prevalence of areas of disturbance, and transportation networks (Gaertner et al. 2017). Human population density often correlates with species richness in a variety of ways. For example, the species richness of invasive woody plants increases with human population, with areas heavily populated with humans serving as sources for invasive plants that have escaped cultivation (Gaertner et al. 2017; Boyce 2010). Most of the ornamental plants sold in nurseries and used in urban and suburban landscaping are non-native, with plants from Asia and those from families with tendencies to become invasive being more heavily represented (Harris et al. 2009; Reichard & White 2001). The vast majority of the plants we use in horticulture, agriculture, and forestry are not native to the areas in which they are planted and some have escaped cultivation and become invasive (Reichard & White 2001). These invasions come with significant consequences. Invasive plants, animals, and fungi are the leading cause of endangerment to native plant species second only to habitat loss and degradation (Reichard & White 2001).
Impacts of Invasive Species
Invasive plants, along with climate change, are two of the most far-reaching and serious causes of ecosystem disturbance and loss of biodiversity worldwide (Bradley et al. 2010). Invasive plants are one of the largest challenges facing natural resource management. They can alter ecosystems, degrade ecosystem services, and impact water and soil quality (Pearson & Ortega 2009). Invasions by nonnative plants also result in the loss of habitat and the loss of forage for wild herbivores and livestock and impact ecosystem services for humans, including agricultural and forestry resources (Pearson & Ortega 2009). The economic cost of invasive plants on managed lands in the United States is billions of dollars per year (Bradley et al. 2010).
In addition to these impacts, invasive plants can decrease species richness, alter nutrient cycling, compete with native plants for pollinators, spread disease, impact fire regime, and interbreed with native plants (Pearson & Ortega 2009; Boyce 2010). Invasive plant species compete with native plants, including threatened and endangered species, for space, nutrients, sunlight, and other resources. This competition leads to changes in plant populations and a decrease in plant biodiversity (Boyce 2010; Bradley et al. 2010; Rockwell-Postel et al. 2020). Animals are impacted by invasive plants through feeding interactions and changes to the amount of available food, structural changes to ecosystems, such as the reduction of habitat, and poisoning via toxic plants (Rockwell-Postel et al. 2020).
Recent studies show that the southern United States is an area of high concern due to the spread of nonnative invasive plant species and their displacement of native species (Oswalt & Oswalt 2011). Southern forests are experiencing invasions of nonnative plants that reduce forest productivity, degrade biodiversity, and impact wildlife habitats (Miller et al. 2015). There are currently more than 50 non-native plants aggressively invading the 13 Southern states and more with the potential to become invasive (Miller et al. 2015; Oswalt & Oswalt 2011).
Impacts of Climate Change on Plant Invasions
Plant invasions and climate change are major environmental issues threatening ecosystems globally. The two issues may also have important interactions, and climate change is expected to increase both plant invasions and their impacts. Climate change facilitates plant invasion by changing environmental conditions, increasing environmental disturbances via extreme climatic events, and human responses to climate change impacts (Turbelin & Catford 2021). While all plants will presumably be affected by climate change, the environmental impacts of climate change will likely be favorable to invasive plants at the expense of native plants (Turbelin & Catford 2021). Studies indicate that higher atmospheric concentrations of CO2 increase the competitiveness of invasive plants over native species (Bradley et al. 2010). Further, the ways in which invasive plants impact the environment, such as through changing soil properties and chemistry and altering hydrology and fire regimes, can contribute to or intensify the effects of climate change (Turbelin & Catford 2021).
Plant populations are expected to migrate to higher latitudes and elevations as climate change intensifies (Hickman & Lerdau 2013). As winters continue to warm, more species will expand their ranges north (Hickman & Lerdau 2013; Rockwell-Postel et al. 2020). Warmer summers will make lower latitudes and elevations less hospitable and less habitable. In addition to native range shifters, invasive plants are projected to shift their ranges to higher latitudes and elevations as well (Hickman & Lerdau 2013).
This climate change-driven migration will create additional hotspots where invasive species establish and spread. The Southeast U.S. has been an area of high risk for invasions for decades and the Northeast is expected to become a hotspot as warming increases and the climate continues to change (Hickman & Lerdau 2013; Rockwell-Postel et al. 2020). The negative impacts of invasive species in the Southeastern states could expand into the Northeastern states as invasive species expand their ranges (Rockwell-Postel et al. 2020). It is expected that climate change will shift the risk of plant invasions several hundred kilometers north by the end of the century (Bradley et al. 2010).